add a right and left border to the content area

Tricky business.
You need to subtract the border width from the overall size of the content area (which is computed based on the settings you chose under Layout).
In your case, #main has a width of 1100px, so this custom CSS should do the trick:
`#main {
border: 0px solid red;
border-width: 0 2px 0 2px;
width: 1096px !important; )`
Change the border width and its colour to your desired values. Just remember to subtract the both border widths from the total width.
You also need to update this width if you ever change your site width from the Layout options.